01 - Line a 9x9-inch pan with parchment paper. Mix confectioners' sugar and cornstarch together, then dust half the mixture generously over the parchment.
02 - Combine gelatin powder and 1/2 cup cold water in the bowl of a stand mixer. Let stand for at least 10 minutes until gelatin absorbs the liquid and becomes firm.
03 - In a saucepan, combine granulated sugar, corn syrup, 1/4 cup water, and salt. Stir gently over medium heat until sugar dissolves completely.
04 - Increase heat to high and bring to a rolling boil without stirring. Attach a candy thermometer and cook until mixture reaches 240°F, approximately 10–12 minutes.
05 - With mixer running on low speed, carefully pour hot syrup into bloomed gelatin. Gradually increase speed to high and whip for 8–10 minutes until mixture is white, thick, and tripled in volume.
06 - Add vanilla extract and mix on medium-low speed for 30 seconds until fully incorporated.
07 - Working quickly before mixture sets, scrape marshmallow into prepared pan. Use a damp spatula to smooth the top evenly.
08 - Sprinkle remaining sugar-cornstarch mixture over the top. Let stand uncovered at room temperature for at least 4 hours, preferably overnight, until completely set.
09 - Turn marshmallow slab onto a cutting board. Cut into squares using a sharp knife dusted with powdered sugar. Toss pieces in remaining sugar-cornstarch mixture to coat all sides.
10 - Keep marshmallows in an airtight container at room temperature for up to 1 week.